How Very Large Diamonds are Cut

The New York Times runs an obituary of Antonio Bianco, a master diamond cutter who was one of the handful of cutters entrusted with very large stones. In the process, the Times describes how these unique and prized stones are coaxed into existence:

Mr. Bianco was among the tiny handful of cutters entrusted with the world’s largest and most unusually colored diamonds, both of which present great challenges to even an accomplished artisan.[…]

A round-cut 5-carat diamond is a little smaller than a dime. Most master cutters pass their entire careers handling diamonds no bigger than 20 to 50 carats — more or less the size of a quarter. For most cutters, a 100-carat stone is beyond contemplation.

Over his career, Mr. Bianco cut about half a dozen diamonds of 100 carats or more. Several of these were for Safdico, the South African Diamond Corporation. Among them were the diamonds known as the “Dream” and the “Golden Star, ” both cushion-cut vivid yellow stones, and the “Flame,” a pear-shaped white diamond nearly the size of a man’s nose. Each is worth tens of millions of dollars, Ms. Esphahani said. Finished, Mr. Bianco’s stones made their way to Graff Diamonds, where they were set into lavish pieces of jewelry. […]

For Mr. Bianco, cutting a large diamond was a protracted courtship that could take nearly a year. Before he began cutting, he spent months studying the stone, slicing and polishing tiny “windows” along its exterior so he could peer into its heart. When the actual cutting started, he might spend a month on a single facet, Ms. Esphahani said.

Mr. Bianco did everything by hand, including tasks now mechanized elsewhere. His finished stones, Ms. Esphahani said, often greatly exceeded the weights that dealers’ state-of-the-art scanning machines had told them to expect.

To give good weight, a diamond cutter must reconcile competing imperatives. On the one hand, he has to excise all flaws in the rough stone. On the other, he must preserve the maximum carat weight possible. (A rough diamond can lose half its weight in the cutting process.) A misplaced cut can mar the stone or even shatter it. Working with very large diamonds only raises the stakes, with the cutter under great pressure not to slip below the vaunted 100-carat mark.

Colored stones bring still more problems. Diamonds occur naturally in a range of hues, from white to pink to orange to yellow to blue. The color can be intensified through judicious cutting. But the heat of the grinding wheel makes diamond molecules excited and cranky, which can alter the color in misleading ways. Mr. Bianco gave his diamonds frequent rest till they cooled down and behaved themselves again. The result was finished diamonds in striking shades of pink, blue and other hues.